PURPOSE: To determine if improving intensive care unit (ICU) environment would enhance sleep quality, assessed by polysomnography (PSG), in critically ill mechanically ventilated patients. MATERIALS AND METHODS: Randomized controlled trial, crossover design. The night intervention "quiet routine" protocol was directed toward improving ICU environment between 10pm and 6am. Noise levels during control and intervention nights were recorded. Patients on mechanical ventilation and able to give consent were eligible for the study. We monitored sleep by PSG.The standard (American Association of Sleep Medicine) sleep scoring criteria were insufficient for the assessment of polysomnograms. Modified classification for sleep scoring in critically ill patients, suggested by Watson et al. (Crit Care Med 2013;41:1958-1967), was used. RESULTS: Sound level analysis showed insignificant effect of the intervention on noise reduction (P=.3). The analysis of PSGs revealed that only 53% of the patients had identifiable characteristics of normal sleep, whereas 47% showed only pathologic patterns. CONCLUSIONS: Characteristics of normal sleep were absent in many of the PSG recordings in these critically ill patients. We were not able to further reduce the already existing low noise levels in the ICU and did not find any association between the environmental intervention and the presence of normal sleep characteristics in the PSG.